Re: [APML] Strange star image

[Aplanatic@aol.com]

Howdy list,

It's nice to see some posts from old friends...  I've been gone a while.  :-)

I think Herm's got the right idea.  It looks like your mirror has a bit of turned down edge.  This is not due to the bevel.  When the mirror was figured the edge was polished a wee bit more than it should have been, a very common problem even with amateur telescope makers.  This edge defect causes a very small fraction of the light to be diffracted far from the center of the star, thus the flare.  As Herm notes, masking the edge of mirror can reduce the flare considerably.  You might have to mask it by more than an 1/8".  If possible, at first try using cardboard masks of various sizes to see which mask opening reduces the problem to a reasonable level.  If I had to guess, I'd mask off about 6 mm all the way around.  BTW, this has worked on another Vixen 8" with the same problem.

Why is the flare asymmetric?  I'd guess that Capella was not in the center of the field, in other words, not on-axis for this exposure.  The turned edge in conjunction with natural off-axis coma will produce bright star images like the one you've shown.  If Capella was in the center of the field, then the system is 1) out of collimation, or 2) the mirror is astigmatic or 3) the mirror is pinched or 4) the secondary is not flat or ....

Dave Rowe

P.S.  Bobby, that's one nice mosaic that you put together.  It's good to see that you got TP working.  -- my best, DR



> >please take a look at this test image I took yesterday with my new 8"
> >f/4 Newton Vixen R200SS:
> >
> >http://home.t-online.de/home/a.roerig/test/r200ss_test_01_aus.jpg
> >
> >What could cause the flare of the star (Capella) to the left side? Could
> >it be that the mirror is to tight within his mirror cell? Collimation
> >should be ok. Any ideas?


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